The desired properties of tissue, including softness, bulk, stretch and absorbency, are achieved by the means of a steam-heated Yankee dryer cylinder and a doctor blade. The wet fiber web is largely dewatered at the pressure roll nip where the sheet is transferred to the Yankee surface. At this point the paper web has 35-40% consistency. The sheet is further dried on the hot Yankee cylinder to 90-97% consistency and removed with a doctor blade. The mechanical action of the blade results in a disruption of the fiber-fiber bonds and a formation of a microfold structure within the sheet. This process is referred to as creping.
In order to develop the crepe,.the paper web has to adhere to the surface of the Yankee dryer. The adhesion is provided by spraying an adhesive on the dryer. Most common Yankee adhesives are synthetic polymers such as polyaminoamide-epichlorohydrin resins, polyamine-epichlorohydrin resins, polyvinyl alcohols, polyvinyl acetates, polyacrylamides and polyethers. Various low molecular weight compounds, oils and surfactants, are used to modify the Yankee coating properties.
A creping adhesive comprising a mixture of high molecular weight and low molecular weight polyaminoamide epihalohydrin resins, PAE-I and PAE-II, respectively, is described in U.S. Pat. No. 6,277,242.
The tissue industry is interested in manufacturing very soft tissue (premium grades) that requires creping at very low sheet moisture levels (<3%). The conventional creping adhesives tend to become hard and less rewettable upon the extensive drying that is required for low moisture creping. The hard coating results in blade chatter and loss of adhesion, which can result in blade wear and non-uniform creping. Thus, there is a great demand for a creping adhesive that remains soft and rewettable under low moisture creping conditions.
Traditional thermosetting adhesives, very similar to permanent wet strength resins, form very brittle coatings and work better at high moisture (>6%) creping conditions. Some thermosetting adhesives with lower levels of cross-linking are much more universal and can be used for creping over a wider range of sheet moisture, 3-7%. Non-thermosetting resins are generally easier to manage and provide better sheet control when creping at lower moisture levels. Modifiers, e.g. polyols such as glycerol, can further be used to adjust the level of adhesion and to improve softness and rewettability of the coating. However, even with the addition of modifiers, conventional adhesives become too hard and their use is still limited to moisture levels of >3%. Accordingly, there is an ongoing need for improved creping adhesive compositions.